The invention generally relates to electronic depth finder systems in which depth can be displayed usually by LED's or neon lamps and which a flasher display type of fish finder, depth alarm and scale expanding capabilities are employed. Generally such systems employ analog and digital techniques for processing ultra sonic pulses produced from a transmitter and echoed back from a sea floor or other objects within the travel path of the pulses. The time between the transmitted pulse and an echo or reflection of that pulse has been used to establish a depth quantity value, since the time of travel of the pulse or pulses through water is known.
In addition, such systems generally employ delayed receiver recovery (DRR) techniques, such as "TVG" (Time Varied Gain), "STC" (Sensitivity Time Control), and "SIS" (Subsurface Interference Suppressor), etc. All of these circuits operate on the same principal, namely, reducing the receiver's sensitivity during the transmit period to thereby allow the sensitivity to return to the normal level at a rate that prevents the depth finder from responding to weak echoes caused by minute particles and air bubbles always present in the area just below the surface of any body of water. The goal of these circuits is to prevent the depth finder from responding to false subsurface interference, yet allow it to display true targets. The premise on which these circuits are based is that true targets will yield a stronger return echo than false targets. Although this is generally true, the greater depth of true targets attenuates the echoe's strength. Thus, without a DRR type of circuit, the true/deeper target echo may very well be the same strength or even weaker than the false subsurface echoes. However, a true/shallow target echo will be stronger than false subsurface echoes. Thus, the approach of varying the receiver's sensitivity vs depth is an acceptable approach to reduce this problem.
It should be obvious from the above that optimising the degree to which the sensitivity is reduced and the period over which normal sensitivity is recovered represents a formidable design task. The following is only a short tabulation of the various factors that must be considered:
______________________________________ 1. Ultrasonic Frequency 2. Transducer Beam Width 3. Detector Resolution 4. Noise Reject Circuit Delay/Resolution a. Bottom Type b. Bottom Reflection Co-efficient c. Depth of Target d. Target Size e. Target Relative Density (compared to water) f. Target Shape g. Presence of Thermoclines h. Characteristics of Subsurface False Targets (size, density, etc.) ______________________________________
Of the above tabulation only four factors 1-4 are adjustable by the designer. All other factors a-h are continuously variable. Present conventional circuits are either fixed, where an attempt is made to come up with one level of gain vs time that is applicable to all conditions, or variable where ONE of the critical parameters, such as level of sensitivity reduction or recovery rate is varied by an operator adjusted control.